The synthesis of higher alcohols from synthesis gas by direct catalysis was recognized in 1923 by Frans Fischer and Hans Tropsch. They reported that a mixture of alcohols, aldehydes, ketones, fatty acids, and esters were formed when the reaction between CO and H2 was performed at pressures ranging from 10 to 14 MPa and at temperatures of 400 to 500° C. in the presence of an alkalized iron oxide catalyst. They named the mixture as synthol and the process as the synthol process.1 In 1930, Frolich and Cryder reported the formation of alcohols higher than methanol by passing syngas over a Zn:Mn:Cr, 1:1.1:1.03 catalyst. They reported that methanol forms from a formaldehyde intermediate and that the higher alcohols form from the methanol through a stepwise condensation reaction.2 In the 1940s, Du Pont developed an alkalized Mn—Cr catalyst to synthesize methanol and higher alcohols from syngas for commercial purposes.3 In the late 1940s, Farbenindustrie et al. introduced the Synol process for the manufacture of alcohols from syngas. This process uses low pressures around 2 MPa with higher productivity of alcohols by modifying the Fischer-Tropsch alkalized iron catalyst.4 Natta et al. reviewed the synthesis of higher alcohols from CO and H2, in 1957 and reported that the synthesis of higher alcohols was always related to the presence of strongly basic substances.5 
Higher alcohols synthesis from CO hydrogenation is of interest as the alcohol mixture is an effective octane number enhancer for motor fuels.6,7 The catalytic systems used for the higher alcohols synthesis reaction from synthesis gas are divided into two groups, based on the product distribution.8 Alkali-doped high temperature ZnCrO-based and low temperature Cu-based catalytic systems produce mainly methanol and higher branched alcohols.9 The second group developed from Fe, Ni, or Co modified low temperature and low pressure methanol synthesis catalysts and alkali-modified MoS2-based catalysts yields a series of linear primary alcohols and gaseous hydrocarbons with Anderson-Schulz-Flory (ASF) carbon number distributions.10 
Alkali-modified MoS2-based catalysts are commercially attractive among different higher alcohols synthesis catalysts, due to their excellent sulfur resistance and high activity for water-gas shift (WGS) reactions.11 The promotion of Pt group metals, especially Rh in Mo-based catalysts, improved activity toward the formation of higher alcohols.12 The Mo promotion over the Rh/Al2O3 catalyst increased its activity favoring the formation of oxygenates.13 Li et al.14 explained that a strong interaction occurred between the rhodium modifiers with the supported K—Mo—O species in the oxidic Rh modified Mo—K/Al2O3 samples and concluded that the coexistence of cationic and metallic Rh stabilized by this interaction may be responsible for the increased selectivity toward higher alcohols (C2+OH). Foley et al.15 suggested that the interaction between Rh and Mo leads to the formation of electron-poor sites that are responsible for the formation of alcohols. Shen et al.16 investigated the promotion effect of Mo in Rh—Mo/SiO2 catalysts in an oxided state and suggested that Mo promotion either leads to the oxidation of Rh and consequent stabilization Rh1+ ions or the coverage of Rh sites by Mo oxides, depending on the interaction between Rh and Mo. Depending on the status of the rhodium species, properties of alkali promoters, nature of the support, and reaction conditions, the rhodium species are capable of catalyzing dissociation, insertion, and CO hydrogenation.17 
The addition of 3d transition metals, such as Co and Ni to MoS2, has a strong promotion effect on the CO hydrogenation reaction.18,19 The promotion of Co (or Ni) on MoS2 leads to the formation of three different phases: MoS2, Co9S8 (Ni3S2), and a mixed Co (Ni)—Mo—S phase.20 The formation of the Co (Ni)—Mo—S phase is related to the electron donation from Co (Ni) to Mo decreasing the Mo—S bond strength to an optimum range, thus significantly increasing the activity of the catalyst.21 The Co-promoted alkali-modified molybdenum sulfide catalysts showed better activity and selectivity of higher alcohols compared to that of Ni.22 The Ni promotion on alkali-modified MoS2 catalysts favors the formation of hydrocarbon as Ni is a methanation component.23 Fujumoto et al.24 found that equal amounts of hydrocarbons and alcohols resulted from the CO hydrogenation reaction over the K/Co/Mo/Al2O3 and K/Co/Mo/SiO2 catalysts. Li et al.10 introduced Co as a promoter to activated carbon-supported K—MoS2 catalysts and found that Co exists mainly in the form of the Co—Mo—S phase at low Co loading and partly in a Co9S8-like structure at high Co content. The addition of Co to alkali-modified MoS2 catalysts enhanced the C1→C2 homologation step that leads to the formation of ethanol as the dominant product.25 Wong et al.26 investigated the incorporation of Rh into reduced K/Co/Mo/Al2O3 catalysts and found that the activity and selectivity of alcohols improved significantly due to the interaction of cationic Rh species with the Mo species.
Catalyst support plays an important role for reactions involving hydrogen as a reactant or product. Supports such as activated carbon,27 clay,28 Al2O3, 29SiO2,30 CeO2,31 ZrO2,32 and combinations of different metal oxides33 have been studied in detail as supports to different catalyst systems for higher alcohols synthesis reactions from synthesis gas. Acidic metal oxide supports, such as Al2O3 and ZrO2, favor the formation of hydrocarbons by suppressing the reaction rate of alcohols and their surface acidity causes deactivation by coke formation.34,35 Concha et al.36 compared the effects of different supports (SiO2, Al2O3, activated carbon, and CeO2) on reduced and sulfided molybdenum catalysts and found that hydrocarbon selectivity on activated carbon-supported catalysts was much less than that of others. Murchison et al.37 found that MoS2 supported on activated carbon showed alcohol selectivity about six times higher than that of the alumina-supported catalyst. Activated carbon has many advantages as a catalyst support because of its large surface area, limited interaction between the support and the active material due to the inertness of the graphitic surface, resistance to acidic or basic media, and stability at high temperatures and pressures.38 However, activated carbon supported catalysts have a microporous structure (pore size<2 nm) that causes pore plugging due to the formation of coke and deactivation of the catalyst, which results in transport limitation in the reaction.39 Also, activated carbons have 10-15% ash content, depending on the nature of the precursor used.40 
Carbon, in the form of multiwalled carbon nanotubes (MWCNTs), is an alternative heterogeneous catalyst support having an inert graphite nature and high temperature stability.41,42,43 